Understanding Setpoints in Temperature Control

In temperature control applications a setpoint is the target value at which a controller attempts to maintain the process variable. This can be achieved by adjusting its control output power (the correcting variable). Controllers have a local setpoint and sometimes remote or other alternative setpoints. Setpoint values are limited by the instrument input range and any setpoint limits.

Remote Setpoint (RSP)

An analog remote setpoint signal can be used to adjust the value of the effective setpoint, as an alternative to using manually entered local setpoints. A controller with remote setpoint has a 2nd auxiliary input that can accept a mA or VDC signal (or in some cases potentiometer or mV signal) which can be scaled and used as the setpoint source. Typical uses for remote setpoint inputs are multi-zone setpoint and master & slave applications and on cascade control slaves.

Alternative Setpoint

Some controllers can have several setpoint sources. Usually there is a local setpoint and one or more alternative setpoints. The alternative setpoints can be additional local setpoints or a remote setpoint input from auxiliary analog input. Selection of the setpoint source can be via the instrument menu or a digital input. Only one setpoint can be chosen as the active at a time.

Active Setpoints

The term Active Setpoint is used to describe the currently selected setpoint of a controller. Some controllers can use the main Local Setpoint or an Alternative Setpoint. The alternative setpoint can be another local setpoint or a remote setpoint sent from an external device. Only one setpoint can be active at a time.

On controllers that incorporate a Profiler / Programmer feature, the active setpoint value is controlled by the profiler function when a profile program is active.

Setpoint Selection

If a controller has more than one setpoint source available, the user can choose whether the main or alternative setpoint will be active. Usually the choice is made via a “setpoint select” parameter in the menu; or via a digital input or by a command given over a serial communications link.

Auxiliary Input

An extra analog input that can be used in addition to the main process input. This can provide functions such as remote setpoints, heater current measurement or valve position indication. Typical signal types are mA, mV, VDC or Potentiometer. These signals are scaled to represent the desired input in the appropriate engineering units. For example, a 4 to 20mA signal might be scaled so that 4mA equals 0.0%RH, and 20mA equals 100.0%RH.

Setpoint Limits

Setpoint limits are the minimum and maximum permissible value for a controller’s setpoint. Setpoints are always limited by the input span, but some models have the facility to restrict the setpoint further. Such limits should be set to keep the setpoint above/below any value that might cause damage to the process.

Setpoint Ramping

Many controllers have the facility to ramp their effective setpoint towards the final target value at a predefined rate. When the setpoint reaches the top of the ramp, a “soak period” begins where the setpoint is maintained at this value.

A deviation alarm is often used with this feature to check that the process is closely following the ramp. Ramping protects a process from rapid changes in the setpoint and the resulting thermal shock as the controller tries to force the process variable to follow. This is especially useful if there is a power-cut, because it guides the rise back to the target setpoint when power is restored.

If you set the ramp rate to 600°/hr and the setpoint to 400°C, and the current temperature is 100°C at power-on, the effective setpoint starts a 100°C and rises towards 400° at 600°C per hour. A similar process occurs when switching back to automatic mode from manual control. 

The exact implementation of setpoint ramping varies with the controller model. Some implement a ramp whenever the setpoint value is adjusted, others only do so when the active setpoint is changed (e.g. from local setpoint 1 to local setpoint 2). Also some models have an adjustable time for the soak, after which the control outputs are disabled. Others have an indefinite soak following the ramp.